This invention relates to thermal ink jet printing systems and, more particularly, to an improved printhead design incorporating multiple levels of interconnection and ballast resistors for the resistive thermal energy generators.
Side shooter thermal ink jet printers are well known in the prior art as exemplified by U.S. Pat. No. 4,601,777. In the systems disclosed in this patent, a thermal printhead comprises one or more ink-filled channels communicating with a relatively small ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. A plurality of heating resistors are located in the channels at a predetermined distance from the nozzle. The heating resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
In typical applications, ink droplets can be ejected at a rate of 5 kHz, giving rise to process speeds of up to 15 inches per second at 300 spots per inch (spi) printing resolution. To achieve practical print speeds, it is necessary to print with arrays of about 20 or more nozzles which are constructed preferably at the same pitch as pixels to be printed. Printers with small nozzle count use a scanning printhead and typically have print speeds of 1 page per minute (ppm). In order to print at speeds above 10 ppm, it is necessary to build a page width print bar which typically contains several thousand jets. With process speeds of 15 inches per second, it is possible to print over 100 ppm with such architectures at 300 spi resolution. Therefore, to enable high throughput thermal ink jet print engines, page width print bars are essential.
The performance of the printhead depends strongly on the distance between the heating resistor and the nozzle. Drop size, drop velocity, and frequency of ink droplet ejection all depend on the distance between the heating resistor and the nozzle. Three hundred spot per inch printing performance is optimized when the heating resistor begins about 120 ppm behind the nozzle. The proximity of the heating resistors to the nozzle, coupled with the high packing density necessary for high density printing have the implication that electrical front lead connection to one end of the heating resistors must be made across the front of the heating resistor array. The short distance from the nozzle to the heating resistor requires the front lead to be narrower than 120 ppm. For arrays of jets designed to operate up to a couple of pages per minute, the configuration where one end of the heating resistors is connected in common from both ends of the array is satisfactory. The problems with wider arrays, such as page width, emerge because of the heating resistor energy requirement for printing, coupled with higher common lead resistance.
As mentioned previously, the thermal ink jet process uses rapid boiling of ink for drop ejection. Electrical heating pulses are applied for a few microseconds and must dissipate sufficient energy in the heating resistor to raise its surface temperature to about 300.degree. C. in order for bubble nucleation to occur. Typical energies required for drop ejection are between 10 and 50 microjoules (.mu.j), depending on the transducer structure and design. It is necessary to apply the energy within a short time, such as 3 to 5 microseconds. Therefore, about 8 watts are being dissipated during the heating pulse. The current necessary for heating depends on the resistance value of the transducer. If a resistance value of 200 ohms is chosen, then 200 mA of current is required and the device operates at 40 V. It is desirable to use high operating voltages so that currents are lowered, but high voltage adversely effects heating resistor lifetime. Therefore, a moderate voltage such as 40 or 60 V is chosen.
Another requirement of the circuit used for thermal ink jet printing is imposed by the drop ejection frequency (.apprxeq.5 kHz or a period of 200 .mu.sec) and the heating pulse length of.apprxeq.5 .mu.sec. In the 200 .mu.sec period, only 40 jets can be fired. However, monolithic printheads can be made using the present semiconductor process technology with about 300 ink channels. Therefore, for maximum efficiency, the printhead must be capable of firing 4 to 12 jets simultaneously. (Of course, the exact number fired during any particular time depends on the document data being printed.)
Another important consideration is the uniformity of the drops ejected from the various channels of a printhead. In order for the threshold for drop ejection to be the same when one jet or all jets are fired, the lead which connects the heating resistors to the power supply should have negligible resistance in comparison with the resistive elements. Tests have shown that a difference of only 1% in the power delivered to a heating resistor produces on the printed page a visible difference in drop size. Another factor contributing to nonuniform drop size occurs in the case in which MOS drive transistors, fabricated on the printhead, are used to supply current pulses to the heating resistors. The parasitic resistance of the front common can lead to variations in the V.sub.gs of the drive transistors.
For the case just discussed, 4 simultaneously fired jets have a total resistance of 50 .OMEGA.. An array of two hundred jets with a resolution of 300 spots per inch is 0.666 inches, or 17,000 .mu.m, long. The width of the metallization in front of the heating resistors is.apprxeq.100 .mu.m, so there is 170 .sub..quadrature. of metal. For typical commercial metal thickness (1.25 .mu.m) and deposition techniques, aluminum has a sheet resistance of 0.032 .OMEGA./.sub..quadrature.. Therefore, the common metal lead has an end to end resistance of 5.5 .OMEGA.. By connecting the metal on both ends, the resistance seen by the middle 4 heating resistors is 1.35 .OMEGA., or 2.7% of the heating resistor resistance.
From the above example, it can be seen that as the number of jets within a module grows, more jets must be simultaneously fired and the parasitic resistance effect caused by the aluminum common connection increases. The practical upper limit before an alternative approach needs to be considered is a consequence of the overvoltage which will be applied when only one heating resistor is fired, given that all elements need to fire if selected. Overvoltage increases power dissipation, shortens element lifetime, and causes drop nonuniformity. For the devices considered here, 4 to 6 simultaneously fired jets is the maximum which is practical.
In addition to the problem of the parasitic resistance effect, a second problem when using the aluminum common connection for wide arrays is the connection of the common between a plurality of chips which have been butted together to form the wide array. In order to butt together arrays of modules, each module must terminate so the spacing between it and its neighbors does not give rise to a noticeable and undesirable stitch error. It is well known that printing irregularities as small as 25 .mu.m can be seen. Therefore, the modules must be within a few micrometers of their correct location. As an example, at 300 spi, 84.5 .mu.m is the pixel spacing. The thermal ink jet channel structure takes up about 65 .mu.m, leaving.apprxeq.20 .mu.m for creation of a butted joint. The 20 .mu.m joint can not deviate more than.+-.5 .mu.m before perceptible image quality degradation occurs. There is insufficient space at the ends of the module to make a low resistance connection to the common power lead which runs along the front edge of the module. Even when single modules containing many heating resistors are fabricated and front common leads can be brought out at the ends of the array, it may be desirable to make additional interconnections to the common in order to avoid parasitic voltage drop when many elements are simultaneously fired.
One approach to overcoming the above-mentioned limitations is disclosed by U.S. Pat. No. 4,887,098, which shows the common connection modified by forming two commons and interconnecting them with leads that pass between adjacent heating resistors. By providing a second common, the first common located between the heating resistor and nozzle can be made relatively narrow enabling the heating resistor to be located at an optimum distance upstream of the nozzle without being restricted by the width of the unmodified wider common. The heating resistor are connected to the heating pulse source by a low resistance structure which crosses over, or under, the second common. In one embodiment the low resistance crossover structure is a heavily-doped polysilicon layer and the second common is aluminum. Other possible combinations shown include an n+ diffusion in a p-type wafer and aluminum; refractory metal silicides and aluminum, either a single or double level metal process. These embodiments have the effect of decreasing the parasitic resistance associated with the single common and providing additional space to make the interconnection between butted-together chips.
The approach disclosed in U.S. Pat. No. 4,887,098 generally performs well in reducing the affects of parasitic resistance of the first common. In particular, the use of a second common reduces the resistance seen by the middle four heating resistors in an array. However, since the space between adjacent heating resistors is relatively narrow, the leads that interconnect the first and second commons are themselves relatively narrow, and are prone to parasitic resistance. The parasitic resistance of the interconnecting leads can result in the resistance seen by the middle four heating resistors being significantly greater than the resistance seen by an end four heating resistors.